I. Field of the Invention
This invention relates to a double sweep streak camera device suitable, for instance, for separately measuring the plural waveforms of optical phenomenon which occur repeatedly with substantially the same period.
II. Background Information
A streak camera has been known as a device for measuring the variation in intensity distribution of a light emission which changes at high speed.
The streak camera includes an electron tube which is called a streak tube. The streak tube has a photocathode at one end, a phosphor screen (layer) at the other end and a pair of deflecting electrodes are disposed therebetween.
When a light beam is applied to the photocathode of the streak tube, the photocathode emits photoelectrons as a function of the incident light beam, thus forming a photoelectron beam which changes with time with changes in the intensity of the incident light beam.
When the photoelectron beam is passed through the electric field formed by the deflecting electrodes while advancing towards the phosphor screen, it is caused to sweep on the phosphor screen in one direction. As a result the change in time of intensity of the incident light beam appears as the change in luminance of the photoelectron beam in the direction of sweep (i.e., the direction of the time axis) on the phosphor screen. This is a so-called "streak image.38 The streak image is photographed with a camera or detected with a TV (television) camera, so that the distribution of brightness or luminance of the output image in the direction of sweep can be quantized for measurement of the change in time of intensity of the light beam.
There are two types of streak camera so called "single sweep streak camera" and "synchro-scan streak camera". Single sweep streak camera is used to measure a low repetition rate phenomena or a single event phenomena.
When the light beam to be measured is a repetitive pulsed light beam which occurs with the same waveform and with the same period, the sine wave voltage whose period is coincident with that of the pulsed light beam and whose phase is in constant relation with that of the pulsed light beam is applied to the deflecting electrodes of the streak tube. In this case, the streak images, having the same light emission distribution in the direction of sweep (i.e., the direction of time axis), can be laid one on another at one position on the output phosphor screen. If the streak images are laid one on another n times, the streak image brightness (or optical energy) on the output surface is substantially increased by a factor of n, and therefore even a considerably weak light emission can be observed with a satisfactory S/N ratio.
The high repetition laser employed usually is a mode locked dye laser having a repetition frequency of about 100 MHz. In this case, for instance in a one-second measurement, the integration can be made 100,000,000 times. The synchro scan streak camera is based on the above-described principle.
FIG. 6 is a block diagram of synchro scan streak camera with its streak tube sectioned along the plane which includes the optical axis.
As shown in FIG. 6, a cylindrical housing 81 has a photocathode 82 formed on the inner surface of its one end, and a phosphor screen 87 formed on the inner surface of its other end which is transparent. A voltage which is lower than the ground potential is applied to the photocathode 82 from a power source E.sub.2.
A mesh electrode 83 is disposed adjacent to the photocathode 82. In order to accelerate photoelectrons emitted from the photocathode 82, a voltage higher than that of the photocathode 82 is applied to the mesh electrode 83 from a power source E.sub.1. A focus electrode 84 is arranged between the mesh electrode 83 and an anode plate 85 having an opening at the center. The anode plate 85 is grounded. A voltage is obtained by using voltage divider 89 is apply some part of the voltage of source E.sub.2 to the focus electrode 84 so that the focus electrode 84 serves as an electron lens which focuses the photoelectrons emitted from the photocathode 82 on the phosphor screen 87.
A pair of deflecting electrodes 86a and 86b made up of a pair of flat plates are disposed adjacent to the anode plate 85. A periodically varying voltage is applied across the deflecting electrodes by a deflecting voltage generating means 88.
FIGS. 7A, 7B and 7C show a graphical representation to assist in explaining the operation of the synchro scan streak camera which is described above. In an ordinary synchro scan streak camera, the deflecting voltage generating means 88 produces a sine wave voltage as indicated in FIG. 7B. The parts p.sub.1 -.sub.1, p.sub.2 -q.sub.2 . . . and p.sub.n -q.sub.n of the sine wave voltage change from positive to negative are used to deflect the electron beam from the upper edge to the lower edge of the phosphor screen 87.
The deflecting voltage is selected so that its frequency is the same as the repetitive frequency of a light beam to be measured, and its phase is in synchronism with the period of the beam.
In order to observe the light emission phenomenon shown in FIG. 7A, a sine wave voltage as shown in FIG. 7B is applied across the deflecting electrodes 86a and 86b. This sine wave voltage which has a repetitive period can be generated synchronous in phase with a laser beam for exciting an object to be observed for instance. FIG. 7C shows the luminance distributions in the direction of the time axis on the phosphor screen 87 which are produced when the phosphor screen 87 is swept with the electron beam.
Assuming the optical intensity of the object under observation is low, the changes in the luminance distribution on the phosphor screen 87 which is provided at the first sweep with the part p.sub.1 -q.sub.1 will be quite small as shown on screen (1) of FIG. 7C and often will not be detectable with the naked eye.
As the above-described operation is repeated, the luminance distribution becomes clear as is apparent from screens (2) and (3) of FIG. 7C. Theoretically, when the sweep is repeated n times, the luminance is approximately n times as great as that provided on the first sweep.
If the light beam under measurement is emitted for the sweep return periods s.sub.1 -t.sub.1, s.sub.2 -t.sub.2, . . . and s.sub.n -t.sub.n of the sine wave sweep voltage synchronous with the period T, shown in FIG. 7B, the streak image formed by the parts s.sub.1 -t.sub.1, s.sub.2 -t.sub.2, . . . and s.sub.n -t.sub.n will lie on that formed by the parts p.sub.1 -q.sub.1, p.sub.2 -q.sub.2, . . . p.sub.n -q.sub.n. However, these streak images are reversed in the time axis direction on the phosphor screen. Therefore, in this case, the images do not add and the measurement cannot be accomplished.
The above-described difficulty can be eliminated by employing a circular scan system such as is shown in FIG. 8. In FIG. 8, parts corresponding functionally to those which have been already described with reference to FIG. 6 are designated by corresponding reference numerals or characters.
The streak tube has, in addition to the above-described streak deflecting electrodes 86a and 86b, another pair of deflecting electrodes 89a and 89b which deflect the electron beam in a direction perpendicular to the direction of deflection of the deflecting electrodes 86a and 86b.
The conventional circular scan system is essential to measure the change with time of a single phenomenon. In general, a light beam incident to the photocathode 82 is focused like a spot, and the photoelectron beam emitted from the spot is deflected to sweep the phosphor screen by the deflecting fields which are formed by applying sine wave voltages which differ in phase by 90.degree. from each other to the two pairs of deflecting electrodes.
FIG. 9 is a diagram showing the output of the streak tube as viewed on the phosphor screen 87. As shown in FIG. 9, the sweep images appear circular; that is, the circular scan system is free from the above-described difficulty. Accordingly, the same repetitive light emissions can be observed as repetitive sweeps on each complete circular scan.
When a pulsed light beam's luminance or brightness is measured according to the synchronous scan system which has been described with reference to FIGS. 6 and 7, a number of problems take place because the streak images cannot be added to improve the S/N ration.
In the case of a specimen generating a fluorescence whose period is longer than half of the period of the sweep voltage employed, the skirt of the fluorescence spreads to the return sweep period, and the streak images formed by the sweeps in the opposite time direction lie on each other. Therefore, the accurate fluorescent period cannot be measured.
Furthermore, if, in measurement of a semiconductor laser beam generated with a period which is just a fraction of one period of the sweep, the laser beam will be generated also in the return sweep period. The streak images will lie on each other on the output surface of the phosphor screen 87. Thus, in this case also, the measurement cannot be made.
As was described above, these problems can be solved by the circular scan system. In order to obtain quantitative data from the streak image, it is necessary to detect the output image with a TV (television) camera.
FIG. 10 shows a streak image obtained using a linear sweep. FIG. 11 is graphical representation indicating the intensity distribution of the streak image of FIG. 10 on the time axis. In the ordinary linear sweep, the TV camera operates in such a manner that the linear time axis is parallel with or perpendicular to the direction of scan of the image pickup tube. On the other hand, in the circular sweep, the operation is considerably more intricate.
If, as in time division spectrophotometry, a linear sweep is performed with various wavelength rays arranged perpendicular to the direction of sweep, then streak images according to each wavelength as shown in FIG. 12 can be obtained. Therefore the data can be readily obtained by detecting and showing the images with a TV camera. On the other hand, using a circular sweep for various wavelengths, streak images are formed as shown in FIG. 13.
A phenomenon called "afterglow" occurs with the fluorescent surface of the streak tube for several tens of microseconds. Therefore, in the case of light emitting phenomena occurring at a high repetitive frequency such as 100 MHz, the streak images are overlapped on the fluorescent surface and therefore cannot be distinguished from one another. It is difficult to read the secondary streak images on the fluorescent surface at a high speed of 100 MHz.
In the single sweep streak camera device, the maximum repetitive frequency of sweep is of the order of 1 KHz, and therefore, in the case where the light emitting phenomenon occur at a high repetitive frequency such as 100 MHz, it is impossible to observe each of the output pulse waveforms which occur successively in response to the light emitting phenomenon.